Microgravity accelerations experienced onboard a space vehicle during flight are vector quantities, which comprise a magnitude and a direction, resulting from numerous forces acting on the vehicle. These accelerations have many sources, including residual gravity, drag, orbiter rotation, vibration from equipment and crew activity. The equivalent acceleration vector at any location in the orbiter is a combination of many different sources and, thus, a complex vector quantity changing over time.
Many experiments conducted in microgravity conditions are extremely sensitive to slight changes in microgravity acceleration. For example, experiments involving gravity-dependent fluid phenomena, such as buoyancy or sedimentation, and experiments involving crystal growth, can be greatly affected by microgravity accelerations. Even acceleration as low as 1 micro-g continuously acting in the same direction could affect certain classes of experiments. As a result, measurement of microgravity accelerations is often necessary to successfully conduct and evaluate such experiments.
However, measurement of the residual quasi-steady g-vector, which is generally the vector sum of aerodynamic drag and gravity gradient accelerations, has been a difficult task. In fact, NASA did not successfully measure quasi-steady acceleration until STS-40. The difficulty is caused, in part, by various mechanical and crew operations that excite the normal vibrational modes of the spacecraft and produce a wide spectrum of periodic accelerations (g-jitter) with amplitudes on the order of milli-g's that ride on top of the micro-g quasi-steady accelerations. Since the g-jitter arises from internal forces and, therefore, must time-average to zero, it should be possible to time-average out the oscillating component of acceleration and recover the quasi-steady component. However, a simple calculation will show that, if the oscillating component has amplitude A and period .delta.t, the uncertainty in the time average taken over time P will be .+-.A .delta.t /2.pi. P. Thus, if one expects to extract a quasi-steady component, when there is an oscillating component that is 3 orders of magnitude larger, the integration time will have to be .about.1000 times longer than the period of the oscillating component in order to obtain any reasonable accuracy. In other words, the data collection that is required to accurately separate the oscillating component from the quasi-steady component is unreasonably burdensome.
Instrument bias is another problem that must be overcome to effectively measure microgravity acceleration. Simple mass-spring accelerometers, which have been commonly used as the basis for many of the accelerometer systems flown in support of microgravity experiments, do not always return to the same null position when acceleration is removed, resulting in an instrument offset that affects the accuracy of the measurement. This instrument offset is sensitive to temperature as well as previous acceleration history. Therefore, no matter how carefully such an instrument is calibrated on the ground prior to flight, the instrument offset will be difference once in space, and can still change with time. This instrument bias is typically on the order of 100-200 micro-g. Attempts have been made to calibrate out this bias by inverting the accelerometer periodically under the assumption that the quasi-steady acceleration does not change during this interval. However, given the limited accuracy that one can obtain from taking a time average over a small interval of time and the fact that one is trying to accurately measure a fraction of a micro-g by subtracting two numbers that are two orders of magnitude larger, such a procedure is problematic at best.
The Orbital Acceleration Research Equipment (OARE) accelerometer, which is based on an electrostatic suspension system, has also been used as a microgravity accelerometer on space flights. The OARE system comprises a charged proof mass suspended electrostatically within a chamber and held in place by an electric field. Voltage is applied to plates surrounding the proof mass in order to maintain the proof mass in a central location within the chamber. The acceleration acting on the proof mass is related to the voltage necessary to keep the proof mass centered. While the OARE instrument is far more sensitive than the mechanical accelerometers, it is also considerably more complex and expensive. Further, it also requires in-flight calibration as the charge on the proof mass may vary. For this purpose, it is mounted on a turntable to either invert it or to apply a known amount of centripetal acceleration to the sensor. The OARE instrument is also sensitive to g-jitter, which must be filtered out electronically or through software.
There remains a need in the art for a microgravity acceleration measurement method and apparatus that requires little or no in-flight calibration and exhibits no loss of accuracy due to g-jitter or instrument bias.